Why the sell-off, and why now? Some theories: The stocks got too overheated; the stocks were over-loved; the stocks got too pricey and had to come crashing down to earth.

Here are a few reasons why the Nasdaq’s latest plunge could be cause for concern:

1. Leaders no more. These once-hot stocks used to be the market “leaders.” But now that they’ve broken hard, the market has no ‘Captain’ or leader to lead the upward charge. In short, market leaders have morphed into laggards.

“They are no longer leaders,” says Bruce Bittles, chief investment strategist at R.W. Baird. “They got so far out of balance with the rest of the stocks in the market” that these (momentum names) were due for a pullback.

2. No more momentum. Momentum stocks have lost theirs, as has the entire stock market this year, says Lance Roberts, chief strategist at STA Wealth.

The risk: The sell-off in the riskier parts of the market, such as the Nasdaq and small-stock indexes, spill over into the broader market indexes, such as the Dow Jones industrial average and Standard & Poor’s 500, says Roberts.

Today, the Dow is down more than 200 points and the S&P 500 is off 1.7%, but both indexes entered the day down just 1% from their record highs.

3. ‘Tis the season for sell-offs. The stock market is heading into its worst 6-month seasonal stretch. There’s an old saying on Wall Street: “Sell in May, and go away.” What’s more, stocks have historically struggled in midterm election years, and there’s an election looming in November.

4. A correction can’t be ruled out. It’s been said a lot — a lot — in recent years: the broad market hasn’t had a 10% drop, or a full-fledged “correction,” since late 2011. It’s due for one, market pros say.

“The market has gone 2 1/2 years without a correction,” says Bittles. “We will have one at some point. You have to be cautious because of that.”

All it would take for a steeper sell-off is a spark, or shock of some kind, adds Roberts.

5. Less liquidity equals less support. The Federal Reserve is still talking about supporting the market with stimulus and low interest rates, but the reality is the Fed is withdrawing stimulus from the system.

As a result, fast money is being yanked out of risky investments and reinvested back into safer ones, such as dividend-paying stocks and U.S. government bonds, says Roberts.

“For years, the market has been basking in the glory of the Fed,” says Roberts. “The Fed has been there. Now we have the Fed taking money away.”